The present invention relates to the field of holographic or optically variable media, a method of providing holographic or optically variable media, and a method for forming holograms or optically variable images on a substrate. More particularly, a multilayered holographic or optically variable material comprising a plurality of embossed panels is provided to print an individually customized holographic or optically variable image onto a substrate by transferring pixels from one or more panels onto the substrate.
Holography or optically variable images have been used in the applications when it is desirable to reproduce the appearance of a one, two or three-dimensional images on various substrates. Reflective transparent, semitransparent, and opaque materials containing embossed holographic images can be used in decorative and security applications. One of the applications of thin films containing holographic or optically variable images is document protection, such as passports, credit cards, security passes, licenses, stamps and the like. Protection is achieved by affixing holographic or optically variable films to the documents, therefore making it very difficult to forge and counterfeit such documents.
An example of a holographic or optically variable film can be found in U.S. Pat. No. 5,781,316 to Strahl et al., which teaches applying a semi-transparent holographic or optically variable transfer foil film to a substrate, such as a security device. The film described in that patent comprises a thermally stable carrier for supporting multiple thermoplastic or thermoset layers. A heat sensitive release layer is applied to the carrier to enable separation of the carrier from the multiple layers of coatings. A wear-resistant transparent topcoat is applied over the release layer to act as an outer surface for the holographic film. The topcoat may be treated or cured in order to increase its tenacity. An embossable layer applied over the topcoat is adapted to retain the impression of an embossed holographic image. A semi-transparent reflective layer of ZnS is applied over the embossable layer for reflecting the embossed holographic or optically variable image and enabling a viewer to see the holographic or optically variable indicia transferred onto the substrate or document. A surface relief pattern is impressed within the reflective layer and the embossable layer to form the embossed holographic or optically variable image. Adhesive and primer or tie layers are also applied for adhering the semi-transparent holographic coating to the substrate.
Although the method described in that patent works to reduce the possibility of forgery of a security document such as an ID card, some risk of forgery or counterfeiting continues to exist. To improve the security of a paper document or a plastic card, a more personalized security hologram or optically variable image may be desirable. Thermal transfer printing is one way of recording and printing variable personalized information on various substrates. A more complex holographic or optically variable image can be created using thermal transfer printers and thermal transfer ribbons, as described in U.S. Pat. No. 5,342,672 to Killey. However, the hologram described in that patent is opaque and therefore not suitable for application on an ID card or other security documents where the personalized individual data needs to be protected from alteration. Accordingly, there is a need for a semi-transparent holographic thermal transfer recording material, which can be applied onto various documents in a individually customized manner for protection and counterfeit prevention purposes.
To print an image by thermal transfer printing using an ink ribbon, the print head of the printer contacts the polymer (or dye diffusion) ribbon and transfers ink to particular locations on the surface of a print medium. The printer head thermally activates the predetermined combinations of heating elements, which are adjacent to the image-forming locations. The ink/carrier structure is locally heated by the heating elements to a temperature at or above the melting point of the ink. In this manner, the necessary amount of ink softens and adheres to the print medium at the predetermined locations to form the image.
Color images are printed with an ink/carrier structure, such as a ribbon, that includes separate regions or panels of differently colored inks, such as the subtractive primary colors, yellow, magenta, and cyan. Color printing is accomplished by sequentially passing the print head along the ribbon, each pass selectively transferring different colored inks to the desired locations on the substrate at predetermined times. Thermal printing ribbons are available with a single black panel, three color panels (yellow, magenta, and cyan), or four color panels, (yellow, magenta, cyan, and black). Such thermal printing process allows a user to create a highly customized color picture or image on the substrate.
It would be desirable to provide a holographic or optically variable printing material and a method for printing or transferring individually customized holographic or optically variable images from the holographic or optically variable transfer material, such as a ribbon, to a substrate.
The present invention is a holographic or optically variable transfer material for application to a substrate, such as a document or device. The first side of the material comprises a thermally stable carrier such as PET, for supporting multiple thermoplastic or thermoset coatings or layers. A release layer is the first layer applied to the carrier to facilitate separation of the carrier from the multiple layers when they are subjected to heat from the thermal print head. A wear resistance topcoat may then be applied over the release layer to serve as the outer surface of the hologram or optically variable image. An embossable layer is applied over the topcoat. A semi-transparent reflective layer of ZnS or possibly Aluminum in the case of a opaque security image is applied over the embossable layer. A tie or primer coating and heat activated adhesive layer are the layers providing the adherence of the transferable holographic material to the chosen substrate. The xe2x80x9cembossmentxe2x80x9d pressed into the embossable layer consists of consecutive sections or panels. Each of the panels is configured in such a way that it reflects incoming light at a certain distinct angle of reflection. A panel reflecting at a predetermined angle can be made by either embossing the panel to reflect at the predetermined holographic reflective angle (using a conventional holographic table), or by digitally creating the optically variable panel by embossing it from a plate or shim produced in a pixel-by-pixel manner (for example, by using a computer controlled origination machine like the Davis Light Machine). Using the latter method, each panel of the present invention comprises the pixels that can later be transferred onto the substrate in the process of forming a customized holographic or optically variable image. All pixels disposed in a particular panel reflect incoming light at the same angle, which angle is different from the angle at which incoming light is reflected by all pixels disposed in another panel. Similarly, if each panel comprises an embossment of a holographically reflective or optically variable angle, then each panel of the present invention will reflect incoming light at a distinct predetermined angle of reflection.
A eye-mark or registration bar to position each panel for registered printing can be provided on either the coated side or the carrier side of the product. The carrier side of the holographic or optically variable transfer material can also include coatings which eliminate blocking of the coatings as well as increase the xe2x80x9cslipxe2x80x9d of the transfer material against the thermal head of a printer.
Using the material and method of transferring of a holographic or optically variable image described above, a personalized or customized holographic or optically variable image can be printed on a substrate by sequentially passing the holographic or optically variable transfer material, such as a ribbon, past the thermal print head of a thermal transfer printer. Each such pass will selectively transfer the material in a pixel-by-pixel manner, or other selective pixel manner, from different panels, so that a custom image analogous to a xe2x80x9cconventually produced optically variable imagexe2x80x9d can be printed on a substrate. For example, the transfer process can involve selectively transferring all the image forming pixels from the panel reflecting at angle xcex11, then selectively transferring all the image forming pixels from the panels reflecting at angle xcex12, and so on as many times as required by the structure of the image or a certain application or program.